This paper proposes a unified four-dimensional spacetime framework based on Boundary Interface Theory, and fundamentally explains the physical origin of the muon anomalous magnetic moment from the intrinsic fourth spatial normal dimension. It clarifies the essential topological difference between electrons and muons as quantum standing waves with different normal embedding depths, and supplements multiple cross-scale physical evidences to verify the rationality of the theoretical system. Both Chinese and English versions of this manuscript are provided for academic reference and open access.